1. Field of the Invention
The present invention relates to a multilayer wiring board formed by stacking a plurality of boards mounting electronic components such as a coil, a condenser and an IC chip.
2. Description of the Related Art
It is possible for a multilayer wiring board having electronic components within its layers (referred to as “built-in component type multilayer wiring board”, hereinafter) to correspond to speeding up of a signal since distances between the components can be reduced. The built-in component type multilayer wiring board can mount the same components as those of a conventional single layer board using an area smaller than that required by the conventional single layer board. Such a built-in component type multilayer wiring board is useful as a printed wiring board unit. Thus, many examinations of the built-in component type multilayer wiring board have been conducted by manufacturers. However, the manufacturing process of the built-in component type multilayer wiring board is complex. Accordingly, it is difficult to obtain a built-in component type multilayer wiring board of high reliability.
The conventional popular built-in component type multilayer wiring board mounts components on a base board having rigidity. Additionally, the multilayer structure is realized by using a resin sheet of such as a prepreg material including glass cloth, for example, and by making the resin sheet form around and bury mounted electronic components.
A description will be given of the conventional built-in component type multilayer wiring board, with reference to FIGS. 1 and 2. FIG. 1 shows a multilayer wiring board 100. In the multilayer wiring board 100, electronic components are mounted on a base board 101 having rigidity, and the base boards 101 are stacked by hollowing out prepreg material corresponding to areas where the electronic components are mounted.
The base board 101 of the multilayer wiring board 100 is formed by completely cured resin including glass cloth and has a preachieved rigidity. On the other hand, generally, the prepreg material is a resin sheet made by mixing glass cloth and adhesive resin, and in a semi-cured state (referred to as “B stage” in this industry). The glass cloth included in the prepreg material interferes with (occupies space for) the electronic component. Thus, a part of the prepreg material is hollowed out so as to secure space for the mounted electronic components. The multilayer wiring board 100 as shown in FIG. 1 is formed by appropriately stacking such base boards 101 and prepreg material, and performing a complete curing process on the prepreg material.
Accordingly, the conventional multilayer wiring board 100 is formed by stacking the base boards 101 each having high rigidity and prepreg layers 102 formed by performing the complete curing process on the prepreg material. A component 105 is arranged on a wiring pattern 103 in a space 104 within the prepreg layer 102. Further, a reference numeral 107 denotes a contact hole penetrating the layers. Inside the contact hole 107, wiring 109 made of copper or the like is formed.
FIG. 2 shows another conventional built-in component type multilayer wiring board 110. In FIG. 2, those parts that are the same as those corresponding parts in FIG. 1 are designated by the same reference numerals, and a description thereof will be omitted. The multilayer wiring board 110 is manufactured in the same way as the multilayer wiring board 100 shown in FIG. 1 and has a structure similar to that of the multilayer wiring board 100. However, filling resin 111 for reinforcement is filled in a space 104 of a prepreg layer 102. Accordingly, the multilayer wiring board 110 requires more manufacturing processes than the multilayer wiring board 100. However, the multilayer wiring board 110 has higher reliability than the multilayer wiring board 100 shown in FIG. 1.
However, as mentioned above, the conventional multilayer wiring boards shown in FIGS. 1 and 2 are formed by stacking the base boards 101 and prepreg layers 102 after manufacturing processes of hollowing out the parts of the prepreg material corresponding to the mounted components. Thus, the number of manufacturing processes increases since additional operations such as hollowing out the prepreg material, positioning the mounted components in the space formed in the prepreg material and the like are required. Further, the operation of positioning the holes in the prepreg material is an operation requiring particular accuracy.
Additionally, the multilayer wiring boards 100 and 110 shown in FIGS. 1 and 2, respectively, are manufactured by a single press process after preparing a plurality of base boards 101 and the prepreg layers 102. Therefore, tests of these multilayer wiring boards are mainly local tests performed on each component before stacking and evaluation tests of finished multilayer wiring boards. Therefore, it is not possible to perform adequate electrical testing.
Additionally, when the finished multilayer wiring board includes a defect, it is difficult to fix the defect. Further, the defective rate increases drastically since the multilayer wiring board includes many layers and components. As a result, the manufacturing cost increases.